In most semiconductor laser diode applications, it is desirable that the laser's threshold current be minimized. Thus, the trend has been toward devices containing (multiple-) quantum-well active regions. By operating at low currents, the devices generate less heat and, consequently, have longer lifetimes. However, in the pursuit of low threshold devices, the quality of the output beam has been somewhat neglected. Typically, the output beam is highly elliptical in shape, having far-field divergences in the planes parallel (.theta..sub.l) and perpendicular (.theta..sub.t) to the laser junction being on the order of 7.degree. and 40.degree., respectively, resulting in a far-field divergence ratio, p=.theta..sub.t /.theta..sub.l, of .about.5.7. For applications, such as, optical recording or laser coupling to an optical fiber, large divergence ratios are highly undesirable. Thus, recently some attention has been devoted to designing lasers with more circular output beams.
Both Yuri et al. (M. Yuri, A. Noma, I. Ohta, and M. Kazumura, `Reduction of beam divergence angles perpendicular to the junction planes by modulating the refractive index profile in AlGaAs laser diodes`, presented at the Fall 1991 meeting of the Japanese Society of Applied Physics) and Cockerill et al. (T. Cockerill, J. Honig, T. DeTemple, and J. Coleman, `Depressed index cladding graded barrier separate confinement single quantum well heterostructure laser,` Appl. Phys. Lett., vol. 59, 2694, 1991) have introduced depressed-index cladding layers into their devices to significantly lower .theta..sub.t. A schematic of an AlGaAs-based laser diode containing these layers in shown in FIG. 1. In the figure is indicated the Al content of the various layers, where 10 refers to the n.sup.+ -GaAs substrate. On the surface of 10 is formed the lower cladding layer 12. Upon 12 is deposited the lower depressed-index cladding layer 14. The index of refraction of this layer is smaller than that of the surrounding layers since the index of refraction of AlGaAs materials is smallest for pure AlAs. On the surface of 14 is formed the lower spacer layer 16, followed by the active layer 18 and the upper spacer layer 20. Upon 20 is formed the upper depressed-index cladding layer 22 followed by the upper cladding layer 24. Lastly, upon the surface of 24 is formed the capping layer 26. Since light avoids low-index regions, the physical effect of the inclusion of the depressed-index cladding layers is to push the transverse-confined waveguide mode both toward the middle and ends of the structure. With greater light intensity present in the lower and upper cladding layers, .theta..sub.t decreases as desired. .GAMMA. remains approximately stationary since light is also pushed towards the middle (active layer) of the structure. More specifically, Cockerill et al. determined that for a broad-area graded-index separate confinement heterostructure device, .theta..sub.t was 27.degree. and 59.degree. for structures with and without the inclusion of the depressed-index cladding layers, respectively. Finally, in the above-referenced copending application of Kahen et al., a single depressed-index cladding layer in the lower cladding region of a ridge waveguide laser diode is implemented. By employing only a single depressed-index layer, the interaction of the field with the rib structure is enhanced, while sufficient spreading of the modal-field is obtained.
This Kahen et al. laser diode is quite effective since its basis is a ridge waveguide laser diode which is simple to manufacture. However, the structure can become unstable for large rib etch depths. More specifically, the modal field at the edges of the rib (where the etched rib contacts the upper cladding layer) can oscillate in a number of different transverse modes for ribs etched deeply into the upper cladding layer, resulting in .theta..sub.l being a strong function of the etch depth and the laser being susceptible to lateral multimoding for small output powers. These transverse modes have nodal structure not in the active region, but in the lower cladding layer and occur for the following reasons. The placement of depressed-index cladding layers nearby the active region effectively takes intensity away from the active layer and redistributes it into the tails of the transverse modal-field distribution. Since light decays quickly at the edges of the rib, the modal-field distribution is cut-off closer to the active layer, causing a greater proportion of the light to leak into the lower cladding layer region. To avoid excessive substrate absorption losses it is then necessary to either increase the thickness of the lower cladding layer or to include a lower depressed-index buffer layer as per commonly-assigned U.S. patent application Ser. No. 923,763 filed Aug. 3, 1992, pending, to T. Hayakawa entitled "Laser Diode". Either way a thick "waveguide" is set-up in the lower cladding layer between the edges of the lower depressed-index cladding layer and the (substrate) lower depressed-index buffer layer, where the nodal structure occurs within this "waveguide" and the dominant mode is determined by the depth of the rib etch. Since the effective transverse index of refraction is smaller for modes with greater nodal structure, the degree of lateral confinement (and the value of .theta..sub.l) is a function of the etch depth, making it difficult to control the shape of the output beam. Thus, the prior art has low manufacturing yields, since many of the devices would fall outside of desired specifications.